This invention relates to a photoionization ion mobility spectrometer for the detection of ionizable chemical species. More particularly, this invention relates to an improved photoionization ion mobility spectrometer system utilizing flashlamp technology as the ionization source.
Ion mobility spectrometry is a technology to detect and identify the presence of an ionizable chemical species, and provide quantitative information. Conventionally, such an ionizable chemical species is ionized using a radioactive source. The ionized samples, which can be positively or negatively charged, are then subject to an electrostatic field which causes the ions to migrate against a counter current flow of a drift gas. Different chemical species migrate with different mobilities and arrive at an ion collector with different elapsed times. Data from such an ion collector can be stored and analyzed to provide information about the ionized chemical species in terms of the elapsed time, and the quantity of the ionizable chemical species contained in the test sample.
Beta particles from a .sup.63 Ni radioactive source generates reactant ions which ionize the chemical species. The use of a radioactive source limits the acceptance of ion mobility spectrometry in the market place due to licensing and waste disposal requirements. Furthermore, an ionizer based on radioactivity provides little specificity for ionization and the ion mobility spectrometer suffers severe interferences, often caused by false positives and false negatives, and matrix effects from components in complex samples. U.S. Pat. Nos. 4,839,143 and 4,928,033 disclosed the use of alkali cation emitters as an ionization source to replace the radioactive source in ion mobility spectrometry. With the alkali cation emitters, ionization can be accomplished in the positive ion mode but not the negative ion mode. Therefore, a large number of electronegative chemical species could not be detected with a ion mobility spectrometer using alkali emitter as the ionization source. Furthermore, significantly high power (greater than one Watt) was required to heat the alkali emitters to the operating temperature (600-800 degrees Celsius).
U.S. Pat. No. 3,933,432 disclosed a low pressure gas filled lamp that excites H.sub.2, Kr, or Xe in a capillary arc discharge to generate the required vacuum ultraviolet radiation for photoionization. Replacing the .sup.63 Ni radioactive ionization source with a photoionization source removes the radioactive hazard. In the '432 patent, the vacuum ultraviolet radiation generated is transmitted through a magnesium fluoride or lithium fluoride window. U.S. Pat. No. 3,904,907 disclosed the use of a helium resonance lamp excited with radio frequency energy. The lamp contains a gettering material to continuously purify the helium. A window, such as aluminum, is provided to pass the desired radiation. U.S. Pat. No. 4,413,185 disclosed the use of a radio frequency inductively coupled discharge lamp with a magnesium fluoride, lithium fluoride, barium fluoride, strontium fluoride, calcium fluoride, or sapphire window. In the '185 patent, finely divided barium is included in the discharge tube as a getter. U.S. Pat. No. 3,699,333 disclosed the possibility of coupling a vacuum ultraviolet lamp to an ion mobility spectrometer. Baim, Eatherton, and Hill in "Ion Mobility Detector for Gas Chromatography With a Direct Photoionization Source", Anal. Chem., Vol. 55, PP. 1761-1766 (1983), disclosed coupling continuously operated photoionization lamps to an ion mobility spectrometer. They used a 10.0 eV (123.6 nm) krypton lamp mounted perpendicular (side-mount) to the direction of gas flow through the cell. Leasure, Fletscher, Anderson, and Eiseman in "Photoionization in Air With Ion Mobility Spectrometry Using a Hydrogen Discharge Lamp", Anal. Chem., Vol. 58, PP. 2142-247 (1986), also similarly disclosed using continuously operated photoionization lamps in an ion mobility spectrometer. They used a 10.2 eV hydrogen discharge lamp mounted coaxial (on axis) to the direction of gas flow through the cell. The results of their experiments showed that the sensitivity and limits of detection were about 1% to 10% of that achieved using the radioactivity source.
U.S. Pat. No. 3,626,181 disclosed the use of a pulsed ultraviolet light source to irradiate an electrode to produce ionized samples. The pulsed ion-producing light source was synchronized with a continuous loop magnetic tape recorder, such that the output signal following each pulse of ultraviolet light was recorded in precisely the same position, and consecutive output signals could be superimposed. Stimac, Cohen, and Wernlund in a government report entitled "Tandem Ion Mobility Spectrometer for Chemical Agent Detection, Monitoring, and Alarm", CRDEC-CR-88082, disclosed an ion mobility spectrometer in which a pulse generator is connected through a pulse transformer to a capillary arc photoionization lamp. Their pulse transformer was triggered synchronously with the ion mobility spectrometer shutter-grid drive circuits with adjustable delays between the grid drive and the lamp pulses. Krypton and xenon lamps were used in their study. Both lamps fired regularly when the repetition period was 15 ms or less. However, they became irregular with a 30 ms repetition, and very erratic with a 100 ms repetition. A 3 second to 30 second interval was needed to initially fire the lamps after being turned off several minutes or longer. In summary, great difficulties were encountered during their attempts to use pulsed photoionization technique in conjunction with ion mobility spectrometer. Furthermore, the purpose of pulsing a capillary arc photoionization lamp was to conserve energy, it was not intended to address specificity or sensitivity.
U.S. Pat. No. 4,551,624 disclosed the introduction of a chemical reagent, such as acetone and/or carbon tetrachloride, into the carrier gas (of an ion mobility spectrometer) to improve the specificity. U.S. Pat. No. 5,032,721 disclosed the addition of a controlled concentration of a dopant substance to the air carrier gas stream prior to application of the carrier gas stream to improve the detection of an acid gas analyte using an ion mobility spectrometer. In both patents, beta-particle ionizing radiation was used to generate product ions from the sample gas introduced into the ion mobility spectrometer by the carrier gas.
Ionization of acetone vapors when they are submitted to photoionization was disclosed by Luczynski and Wincel in an article entitled "Reaction of the Solvated Photon System H.sup.+.[(CH.sub.3).sub.2 CO].sub.n Formed in Photoionization of Acetone". Int'l J. Mass Spectrometry and Ion Physics, Vol. 23, PP. 37-44 (1977), and Tzeng, Wei and Castleman, Jr., in another article entitled "Multiphoton Ionization of Acetone Clusters: Metastable Unimolecular Decomposition of Acetone Cluster Ions and the Influence of Solration on Intracluster Ion-Molecule Reactions", J. Am. Chem. Soc., Vol. 111, PP. 6035-6040 (1989). In these two papers it was concluded that protonated monomer, H.sup.+ (CH.sub.3).sub.2 CO, and dimer, H.sup.+ [(CH.sub.3).sub.2 CO].sub.2 ions were formed.